The present invention relates to a power transfer system for controlling the distribution of drive torque between the front and rear wheels of a four-wheel drive vehicle as a function of various system and operator-initiated inputs.
In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing tractive power (i.e., drive torque) to the non-driven wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. However, the vast majority of such part-time transfer cases include a mechanical "mode" shift mechanism which can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheels to the driven wheels for establishing a part-time four-wheel drive mode. As will be appreciated, a motor vehicle equipped .with a part-time transfer case offers the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions.
Alternatively, it is known to use "on-demand" power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. In one typical application, a viscous transmission device, such as a viscous coupling, is located in the driveline between the driven and non-driven wheels to provide "on-demand" four-wheel drive operation. Modernly, it is known to incorporate the "on-demand" feature into a transfer case by replacing the mechanical mode shift mechanism with a multi-disc clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated state such that drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque "on-demand" to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. One example of such an "on-demand" power transfer system is disclosed in U.S. Pat. No. 4,773,500 to Naito, et al wherein a hydraulically-actuated clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function of the wheel speed difference (i.e., the wheel slip) between the front and rear wheels.
While numerous variations of such hydraulically-actuated "on-demand" power transfer systems are known, they are primarily intended for road use in passenger cars and not for off-road recreational or sport/utility vehicles. In addition, such conventional "on-demand" transfer cases are single-mode power transfer mechanisms which do not provide means for permitting the vehicle operator to deliberately shift between traditional two-wheel drive and part-time four-wheel drive modes of operation. Moreover, most conventional "on-demand" power transfer systems are adapted for use in four-wheel drive vehicles wherein the non-driven wheels are rigidly connected to the transfer case for permitting instantaneous delivery of drive torque thereto when needed. As such, rotation of the non-driven wheels during operation of the motor vehicle causes its entire driveline to likewise be rotatably driven which detrimentally impacts fuel economy and component wear. Thus, the need exists to develop a transfer case for use in conjunction with disconnect-type drivelines and which incorporates an "on-demand" four-wheel drive feature that can be effectively over-ridden upon the vehicle operator selectively shifting the transfer case into either of a two-wheel drive mode or a part-time four-wheel drive mode.